The present invention relates to an improved method of controlling the number of revolutions, i.e., speed, of a rotor by means of a tachogenerator, i.e., tachometer, as well as to an apparatus for carrying out the improved method.
In connection with electronically controlled drives, it is known to generate pulses when a point or points on the driven shaft (rotor) pass a fixed point, with the time sequence of these pulses providing information about the respective rate of rotation of the shaft or the rotor. The thus generated pulse repetition frequency is fed into electronic evaluation circuits which make a comparison, for example, with a desired frequency and derive a voltage to adjust the number of revolutions of the motor.
The pulses are generated by means of a so-called tachogenerator or tachometer, with the type of generation possibly being quite different. For example, mechanical, electromagnetic or optoelectronic tachogenerators may be employed. All generators performing more than one pulse evaluation per revolution have in common that, due to the mechanical arrangement of the individual generator elements, tolerance variations exist during one revolution of the rotor so that the emitted pulse repetition rate is not constant during one revolution of the shaft in spite of a uniform rate of rotation.
If such a generator which is subject to tolerance variations controls, for example, a PLL (phase locked loop) with low internal time constants, the output signal of the PLL during one revolution reflects these tolerances although, with a constant rate of rotation, the output signal should also be constant. Thus, undesirable regulating processes may be initiated during one revolution of the rotor.